The invention relates to a dosing module for a liquid reducing agent, in particular a urea-water solution, for mixing the reducing agent with a gas, in particular air, for onward introduction into an exhaust pipe of an internal combustion engine, the dosing module comprising a dosing valve with a guide tube, comprising a compressed-air duct and comprising a mixing chamber.
The emissions limit values stipulated by the legislators for nitrogen oxides make it necessary to provide in motor vehicles exhaust-gas aftertreatment devices which perform for example a selective catalytic reduction (SCR) of the nitrogen oxides contained in the untreated emissions of the internal combustion engine. Said so-called SCR process for exhaust-gas purification is known from the prior art.
An example of an exhaust-gas aftertreatment device of said type is known from WO 02/07916 A1. In said document, in a dosing module, using a reducing agent, preferably a urea-water solution, to be introduced into the exhaust gases, the liquid reducing agent and additionally compressed air are introduced into a mixing chamber via a dosing valve. In the mixing chamber, there is formed from these a reducing agent-air mixture which is introduced into the exhaust pipe.
The dosing module comprises the dosing valve and the mixing chamber, wherein the mixing chamber is conventionally arranged in a housing composed of corrosion-resistant steel, in which is inserted a profiled seal composed of an elastomer, for example “fluorosilicone”. The seal serves as a check valve, such that the compressed air can pass into the mixing chamber but as far as possible no urea-water solution can pass into the compressed-air region. Said delimitation is necessary because, firstly, in the event of drying of the urea-water solution, urea crystals can be precipitated which can lead to a blockage of a compressed-air line, and secondly, the urea-water solution may, as a result of a possible separation of ammonia, have a highly corrosive effect on parts in the air system.
Furthermore, urea crystals may form on an outer side of the guide pipe at the outlet region during operation as a result of urea-water solution spray. Said urea crystals may dissolve again in urea-water solution. If this does not occur, the urea crystals consequently grow along the guide tube in the direction of the profiled seal and may, over the course of time, fill up significant parts of a compensating region. The resistance for the inflowing air is thereby increased such that failure of the system may occur.